Retarded aging rimmed steel

ABSTRACT

THIS INVENTION RELATES TO A METHOD FOR PRODUCING A RIMMED STEEL WHEREBY THE STEEL&#39;&#39;S AGING CHARACTERISTICS HAVE BEEN RETARDED FOR AT LEAST 90 DAYS. SPECIFICALLY, THE INVENTION IS CONCERNED WITH THE TEEMING PROACTICE IN THE PRODUCTION OF LOW CARBON RIMMING STEEL, WHEREBY APPROXIMATELY .02% VANADIUM IS ADDED TO THE MOLD NEAR THE END OF THE TEEMING, AND THE POURED INGOT IS CAPPED SHORTLY THEREAFTER WITH ABOUT ONE POUND ALUMINUM PER TON OF STEEL. THE SUBSEQUENTLY PROCESSED PRODUCT IS CHARACTERIZED BY 21/2 OR LESS YIELD POINT ELONGATION UP TO 90 DAYS AFTER TEMPER ROLLING AND WITH GOOD SURFACE QUALITY.

United States Patent Oifice 3,558,370 Patented Jan. 26, 1971 3,558,370 RETARDED AGING RIMMED STEEL Robert E. Boni, Middletown, Ohio, assignor to Armco gteel Corporation, Middletown, Ohio, a corporation of bio No Drawing. Filed Aug. 9, 1967, Ser. No. 659,337 Int. Cl. C22c 29/00, 39/02, 39/50 U.S. Cl. 148-36 5 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a method for producing a rimmed steel whereby the steels aging characteristics have been retarded for at least 90 days. Specifically, the invention is concerned with the teeming practice in the production of low carbon rimming steel, whereby approximately .02% vanadium is added to the mold near the end of the teeming, and the poured ingot is capped shortly thereafter with about one pound aluminum per ton of steel. The subsequently processed product is characterized by 2 /2% or less yield point elongation up to 90 days after temper rolling and with good surface quality.

DEVELOPMENT OF THE INVENTION Age hardening in drawing quality rimming steel is manifested as a spontaneous increase in hardness at or near room temperature. In the metal producing industry, particularly in the steel industry, it is generally believed to be associated with the disintegration of a super-saturated solid solution. That is, excessive quantities of certain elements, such as carbon, nitrogen, and oxygen, are retained in solution as a result of the processing steps followed in the manufacturing of the steels. However, as time passes following the final processing, diffusion and precipitation of said elements take placethis phenomenon is considered to be the cause of aging in steels. The precipitation can be further accelerated both by an increase in temperature and by the effects of straining. Thus, while temperature is a factor which can readily be controlled, time and straining through fabrication are unavoidable or necessary to achieve the desired final product. Further, while carbon may cause age hardening with or without straining, the control of nitrogen is of major importance since it diffuses much faster than carbon. Ac cordingly, this invention will beconcerned primarily with nitrogen and its effects on the aging of low carbon steels. The element oxygen is of least importance as it generally will be tied up by the aging controlling agents employed.

The foregoing should not be interpreted as a desire to eliminate completely the phenomenon of age hardening with respect to steels. Quite frequently, added strength and hardness are assets to a fabricated product. However, certain minimum properties are desirable during the fabrication operation as this saves time, money and equipment. Therefore, if a reasonable delay in the hardening process can be built into a steel product, it becomes a more valuable product to the fabricator and to the ultimate user of the fabricated items.

As a result of the present development, a process has been found which imparts a delaying or retarding effect to the age hardening process. This is accomplished by the controlled addition of vanadium and aluminum to tie up the nitrogen only, thus leaving the slow diffusing carbon to bring about the hardening after a sufficient delay.

Prior art attempts to accomplish this result have met with only limited success due to poor or minimum quality in the final product. To further complicate the picture, there are several types of deoxidizing practices, certain of which provide for a better surface quality than others. Rimmed steels are particularly suitable for applications where surface quality is critical. However, rimmed steels are more susceptible to aging than killed steels. Thus, while the aging characteristics of a killed steel are more readily controllable, the same controls are not directly applicable to rimmed steels. Further, where killed steels are produced by the action of large quantities of deoxidizing agents, rimmed steels are produced by the limited addition of same.

Aluminum and titanium have been proposed as deoxidizing agents in the production of killed or semi-killed steels. Vanadium and boron have also been suggested as suitable elements in the control of the aging characteristics. However, to the present time, no one has been able to suitably combine the advantages of vanadium and aluminum and commercially produce a rimmed steel having retarded aging characteristics and good surface quality. In accordance with the above, a primary object of this invention is the provision of a method for producing a rimmed low carbon steel, having delayed aging tenden cies, by the controlled addition of vanadium and aluminum to the steel.

DETAILED DESCRIPTION While this invention will be described with respect to rimmed steel, it is to be understood that it is applicable to and includes capped steels, each of which are processed to a final carbon content no greater than 0.10% by weight. Capped steel practice is a variation of rimmed practice. Though not intended to be limited by the following definition, capped steels are produced by prematurely terminating the rimming action of the poured ingot.

Since the several types of ingots which are produced commercially are dependent for their characteristics upon the extent to which gas evolution is controlled, attention must be given initially to the steel making practice. Rimmed steel usually is tapped without the oxidizing additions to the furnace and generally with only small additions to the ladle. This insures that sufficient oxygen is present in the mold to react with the carbon.

A typical rimming steel suitable for practicing this invention, but prior to teeming, comprises by weight percent the following:

Percent Fe Balance Percent Fe 7 Balance The carbon will drop as a. result of the reaction with oxygen during the running action.

By adding the vanadium at this time, a good distribution of the vanadium throughout the ingot is assured. Since, however, the nitrogen content is higher in the ingot top during rimming as compared to the ingot butt, the vanadium addition will tie up all the nitrogen in the ingot butt and nearly all in the top. As will be explained later, an aluminum cap in the form of shot is employed to complete the nitrogen tie up in the ingot top.

The aluminum cap is applied at the rate of about one pound of aluminum per ton of steel to yield a maximum average content of .02% aluminum. A preferred average content is on the order of .0l% aluminum. The ingot is chemically capped with aluminum -240 seconds after pouring is finished. Preferably, the ingot should be capped in the time period from 3090 seconds following the conclusion of the pouring. In this time, a sufiiciently sound skin has been formed, yet there is enough time to permit distribution of some of the aluminum to the ingot butt. At the conclusion of the solidification, the ingot is processed in the conventional manner which includes the steps of slabbing, hot rolling, coiling, scale removal, cold rolling, annealing and temper rolling. The low carbon rimmed steels produced in the above manner have good surface quality and a 2.5% or less yield point elongation up to 90 days after said temper rolling.

It has been determined that the use of about .04% vanadium will stabilize the rimming steel; however, at this level of vanadium the surface quality of the processed strip is impaired. Specifically, mold additions of vanadium greater than about .02% tend to show an increase in the quantity of seams on the strip. Therefore, it is essential to maintain the vanadium addition between about .01- .02% by weight, or preferably .011.019% by weight. The following demonstrates the effect of vanadium on surface quality in three different heats of drawing quality rimmed steel.

Aluminum, on the other hand, has been found effective in eliminating the aging of steel but it adversely affects surface quality when used in large amounts. By adding aluminum in the prescribed manner, the surface quality in the strip from the butt portion of the ingot is not detrimentally affected because the aluminum capping stops the rimming action thus preventing a large portion of the aluminum from being distributed to the butt portion. To avoid an adverse effect on the butt portion of the ingot, the retained aluminum should be less than .02% and preferably less than about .01% to insure optimum surface quality. The preferred range of aluminum is therefore .005%.0l9%.

Table II is presented to illustrate the dramatic improvements in surface quality when low carbon rimming steels are treated with controlled amounts of aluminum and vanadium in accordance with the invention.

The improvement represents the many-fold reduction in total seams of three strips processed from ingots treated in accordance with the invention, over the average results of aluminum killed drawing steel treated with .04% aluminum.

In addition to the excellent improvement in surface quality, the annealed and tempered, cold-reduced strips 1-3 exhibited aging properties which were retarded for at least a ninety-day period following temper rolling.

EXAMPLE A 315 ton open hearth heat comprising C0.06%, Mn- 0.08%, S0.020%, P0.0l5%, and the balance Fe, where all percentages are by weight, was tapped at 2900 F. into a ladle where ferromanganese was added to raise the manganese content to about 0.3%. The molten metal was then teemed sequentially into a number of 12 ton cast iron molds. As each mold was filled to about of its capacity, approximately 0.4 lb./ ton of a proprietary vanadium additive known as Carvan, nominally containing about 8386% V, 10.5-l3% C, 23% Fe, was added to the mold. With recovery on the order of about 75%, approximately .013% V was retained in the solidifying steel. The teeming was then completed on top of the V addition.

After about 90 seconds following the pouring of each mold, approximately 1 lb. of aluminum shot/ton of steel was added to chemically cap the solidifying ingot. The ingots were then stripped and uniformly heated to a temperature of about 2400 F., then subjected to the following conventional operations:

(a) hot roll to 6" slab,

(b) hot roll to 0.09" strip,

(c) pickled and cold rolled to 0.03,

(d) annealed at 1250 F. for 10 hours, and (e) temper rolled 1%.

A strip processed in the foregoing manner will exhibit excellent surface qualities and a yield point elongation (Y.P.E.) of zero. Typically, after 90 days of aging at room temperature, or under an equivalent time saving thermal treatment, the Y.P.E. will still be zero. On occasion, the yield point elongation may reach a recordable Ivalue with a maximum of 2.5%.

It should be apparent that modifications may be made herein without departing from the spirit and scope of this invention. For example, it is contemplated that by following the present teachings, low carbon rimming steels can be produced by the process known as continuous casting. Briefly, in such a process where the molten steel is intermittently poured from a ladle to a tundish, from which the steel is cast continuously into a water cooled mold, the vanadium may be added to the tundish, while the aluminum is supplied at a constant rate to the mold.

Typically, a continuous cast slab is characterized by a vertical semi-molten zone. This transition zone lies between the molten and solidified portions of the continuous casting. The vertical dimension of this zone has also been defined as the metallurgical height, the top of which lies near the top of the (water cooled mold. Thus, in a manner similar to the conventional casting and solidification of ingots, the aluminum is added at a point near the start of the solidifying process. However, since the distribution of aluminum is more uniform, the maximum average content should not exceed 0.01% by weight.

Since other and further modifications may be made by those skilled in the art, no limitation is intended to restrict this invention except as set forth in the appended claims.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An improved age-hardenable rimmed steel in the form of temper rolled sheet and strip and characterized by good surface quality and by the property of being resistant to an increase in yield point elongation greater than 2 /2 over a 90 day period following temper rolling, consisting essentially by weight less than about .02% vanadium, less than about .02% aluminum, less than about .10% carbon, and the balance essentially iron.

2. The rimmed steel claimed in claim 1 wherein said aluminum is less than about .01

3. The rimmed steel claimed in claim 1 wherein said vanadium is at least .01%.

4. The rimmed steel claimed in claim 3 wherein said vanadium is in the range .011.019%

5. The rimmed steel claimed in claim 4 wherein said aluminum is in the range .005-.019%

References Cited UNITED STATES PATENTS 1,729,378 9/1929 Fry 75- 58X 10 2,768,892 10/1956 Shoenberger 148-12X 6 Morgan 14812X Muta 148-4-12X Lindberg 75-129 Scott 75-123X Epstein 75-123 L. DEWAYNE RUTLEDGE, Primary Examiner J. -E. LEGRU, Assistant Examiner US. Cl. X.R. 

